The present invention relates to a die and to a process for producing a component which is locally reinforced by a porous ceramic insert.
To reduce the component mass, efforts are currently being made to produce relatively large individual components from light metals, for example from aluminum or magnesium, using the pressure die-casting process. This applies in particular to the automotive industry, in which the gear casing and the engine block of motor vehicles are increasingly being manufactured from light metals. However, when using light metals the strength, the resistance to creep and the wear resistance of mechanically loaded partial areas of the components are unsatisfactory in particular in areas which are subject to relatively high temperatures. Consequently, the mechanical load-bearing capacity of light metal components of this type is limited.
A process of the generic type is known from German Patent Document DE 197 10 671 C2. This document discloses a process in which a porous sacrificial body made from a ceramic material (insert) is placed in a defined position in a die and is infiltrated with a molten metal (casting metal) under pressure. The infiltration of the insert with the casting metal leads to the formation of a metal-ceramic composite material (reinforcing element) at the location of the insert. Then, the cast component is heated, so that a reaction takes place between the ceramic material and the casting metal within the reinforcing element, resulting in a composite material comprising ceramic and intermetallic material phases which is superior even to the reinforcing element in terms of its resistance to wear and its strength. However, particularly in the case of local reinforcements, the heating of the component can only be achieved with high technical outlay and high manufacturing costs. Furthermore, process conditions mean that bending stresses may cause damage to the insert during the infiltration.
Japanese Patent Document JP 60130460 A describes a process for producing a composite component which is produced using the centrifugal casting process. A core made from ceramic fibers is placed into a centrifugal die and is supported by holding elements. The holding elements divert the flow of a casting metal past the core, so that after solidification a tube of layered structure is formed, including the core of ceramic fibers and comprising metal at the surfaces. However, a process of this type is not suitable for the infiltration of porous ceramic inserts, since there is not sufficient pressure acting on the insert.
Therefore, the object of the present invention is to provide a die and a further improved process of the above type, so that it is possible to produce light metal components with an improved mechanical load-bearing capacity, in particular an improved resistance to creep, easily and at low cost.
The solution to the object consists in a device (die) having fixing elements for positioning an insert allowing forces which act on the insert to be compensated for by corresponding collinear forces and shielding elements by which the insert is shielded from a principle propagation flow of a casting metal during a casting operation and a process for producing a component with a local reinforcing element made from a metal-ceramic composite material comprising producing a porous ceramic insert from ceramic precursor products; locally positioning the insert in a die which has a runner, a gate, and an impression; filling the die with a casting metal by way of a casting plunger and simultaneously infiltrating the insert at elevated pressure in order to form the local reinforcing element, wherein a preliminary section comprises the filling of the runner and the filling of at least 10% of the impression with the casting metal and wherein a velocity of the casting plunger during the preliminary section is lower than during a filling movement.
The device according to the invention, as described in a preferred embodiment, is distinguished by the fact that, in the die, there are fixing elements which position the insert in a defined way. The fixing elements are designed in such a way that the bending moments which act on the insert are minimized. According to the invention, this is achieved by the fact that forces which act on the insert are compensated for by collinear forces by means of the fixing elements. This means that the force lines of opposite forces lie on a straight line. In addition to the fixing elements according to the invention, the insert is positioned in an impression in such a way that it does not lie directly in the propagation flow of a casting metal. To achieve this, shielding elements are used. Ideally, these shielding elements are components of the impression contour, such as for example edges or walls, which are predetermined by the component geometry. However, it is also possible to design additional fixing elements in such a way that they shield the flow of the casting metal with respect to the insert. Together, the fixing elements and the shielding elements prevent damage to the ceramic insert and thereby reduce the scrap rate in series production of reinforced light metal components.
The insert is preferably positioned in a side of the die which is fixed with respect to a casting machine, since this means that it does not undergo any movement when the die is being closed, which could cause its position to shift. If the geometry of the component and/or the geometry of the die require, it is possible for the insert to be positioned in a moveable side of the die or on a slide. Furthermore, it is possible to position a plurality of inserts in the die, and these inserts may be located in the fixed side and/or the moveable side and/or on a slide.
To minimize the bending moments which act on the insert, it is useful for the insert to be positioned on a wall of the impression. In this case, it is important for the insert to fill up the surface of the die wall in an accurately fitting manner. The die wall is ideally a planar surface.
The definitive fixing of the insert takes place during closing of the die. For this purpose, lugs, pins, edges and/or shielding elements (fixing elements) can be inserted in the tool side which lies opposite the insert (moveable side if the insert is positioned in the fixed side) or on slides.
If the insert is positioned in an accurately fitting manner against the wall of the impression, it is important that no casting metal should penetrate between the insert and the impression wall. This would lead to the insert being lifted off and, together with the action of forces of the fixing elements, would lead to bending moments which would destroy the insert. This can be prevented if, for example, the contact surface between the insert and the impression wall is sealed by edges of the opposite mould side.
In various components, it is necessary for the inserts to be positioned freely in the chamber of the impression. In this case, the fixing is likewise provided by fixing elements. After the impression has been completely filled, the infiltration of the insert takes place uniformly from all sides, i.e. isostatically. Isostatic infiltration has the advantage that the bending moments which act on the insert are reduced to a minimum.
As an alternative and/or to assist the externally acting fixing elements, it is possible to provide the insert with bores and to position it accurately on pins which are located on the fixed side or the moveable side or on a slide. This is advantageous if the design of the component which is to be produced does not locally allow any fixing elements, which are reflected as cavities in the component, to be present in the impression.
The cross section of a casting plunger which delivers the casting metal is generally larger than the cross section of the opening of the impression (gate). The result is that the casting metal is accelerated when it enters the impression at a constant casting-plunger velocity. To protect the insert from the casting metal, it is expedient, in addition to the shielding elements, to maintain a low velocity of the casting metal. In practice, it has emerged that the velocity of the casting metal in the region of the insert should be no greater than eight times the maximum casting-plunger velocity. Therefore, the cross section of the gate should be no less than approximately one eighth of the cross section of the casting plunger.
Components of internal-combustion engines and transmissions are particularly suitable for local reinforcement of light-metal components using the device according to the invention. In these components, very high demands are imposed on the properties of the materials used. Properties which should be mentioned are the bending strength, the modulus of elasticity, the coefficient of expansion and the resistance to wear. Local reinforcements are employed in particular in cylinder liners used in the cylinder crankcase. In cylinder liners, firstly the wear resistance and secondly the rigidity of the liner are of importance. This is particularly important with small cylinder spacings, i.e. a narrow web width, since in this case, without reinforcement, there is undesirable bulging of the liner, which leads to a gap forming between cylinder and liner, through which unburnt fuel can escape (blow-by effect).
Base bearing regions of a crankshaft (e.g. in the cylinder crankcase and/or in the crankcase lower half and/or in the bearing cap) and bearing regions in gear casings represent a further application for local reinforcements. In this case, the increased rigidity of the reinforcement element and the lower coefficient of expansion and the higher resistance to creep compared to the unreinforced light metal can be exploited. On account of the good resistance to wear of the reinforcing elements, it is conceivable that they could also replace the bearing shells in the bearing block.
Further, mechanically loaded components or functional elements which can be reinforced by reinforcing elements are, for example, collecting rods, turbocharger blades or sliding blocks on a transmission shifting fork. Furthermore, brake discs can be reinforced in the region of the friction ring, making use of the resistance to wear of the reinforcing element, which is higher than that of the light metal.
Furthermore, by controlled selection of the starting composition of the insert, it is possible, by applying the device according to the invention, to produce a component in the form of a heat sink with a low coefficient of expansion combined, at the same time, with a high thermal conductivity.
The division of the casting operation into three phases, namely the preliminary section, the filling movement and the recompacting, which is customary in standard pressure die-casting, is employed in modified form in the process according to the invention as described in a preferred embodiment. The three phases are defined by the velocity of the casting plunger as a function of the extent to which the die is filled with the casting metal. A characteristic of standard pressure die-casting is that the casting plunger is moved slowly until the casting metal reaches the impression (preliminary section), and then for the casting plunger to be accelerated (filling movement). However, if there is a porous insert in the impression, it is advantageous for the casting plunger only to be accelerated when the insert has already been surrounded by the casting metal. This prevents damage to the insert and reduces the scrap rate. The extent of filling of the impression when the filling movement commences is dependent on the position of the insert in the component and may be between 10% and 90% in practice, it has proven particularly expedient for the impression to be between 50% and 80% full at the start of the filling movement.
The infiltration of the porous ceramic insert with the casting metal leads to the formation of a penetration structure. This means that the open pores of the insert which are connected to one another via passages are filled by the casting metal. Accordingly, each material phase forms its own three-dimensional framework, and the two frameworks are interwoven with one another in such a manner that a compact body is formed, namely the reinforcing element. One advantage of this type of reinforcing elements over monolithic reinforcing elements, for example made from grey cast iron, consists, in addition to the weight saving, in the fact that there is no defined boundary between the material of the component and the material of the reinforcing element. Rather, the metal of the component is identical to the metal of the reinforcing element and is continuously joined thereto.
Different demands are imposed on the properties of the reinforcing element, and therefore it is expedient, within the context of the invention, to use different raw ceramic powders as precursor products of the insert for different applications. For example, if a high hardness or wear resistance is required, it is advantageous to use titanium carbide or silicon carbide as the raw powder. In the case of components which have to have a high thermal conductivity, silicon carbide or aluminum nitride is a suitable raw ceramic powder. In many cases, the mechanical properties such as strength, modulus of elasticity, resistance to creep or wear resistance are of importance, taking account of the raw-material costs, for the mode of action of the reinforcing element. Depending on these criteria, raw powders such as titanium oxide, spinel, mullite, aluminum silicates or clay minerals are used.
The use of fibers in composite materials generally increases the ductility of a composite material. This stems from the fact that the fibers absorb the energy of cracks, and therefore the composite material has a higher fracture resistance. In this case, the bonding between the fiber and the matrix is particularly important. It has emerged that in the process according to the invention, particularly high fracture resistances are achieved by metal fibers, in particular those based on iron, chromium, aluminum and yttrium. The most favorable thickness for the fibers is in a range between 20 xcexcm and 200 xcexcm, in particular between 35 xcexcm and 50 xcexcm.
Depending on the degree of filling of the die, the velocity of the casting plunger is an important parameter for the process according to the invention. It has emerged that the velocity of the casting plunger during the preliminary section is advantageously between 0.1 m/s and 2 m/s. The velocity of the casting plunger may increase within this range during the preliminary section if this is appropriate for the filling operation. The velocity of the casting plunger during the filling movement is, according to the invention, between 1 m/s and 5 m/s, so that a low velocity in the preliminary section is linked to a low velocity during the filling movement. The optimum velocities are in each case dependent on the geometry of the impression and are accordingly specific to the die. In general, it should be ensured that the lowest possible casting-plunger velocity within the indicated range, which ensures that the component is produced without defects, is selected during the preliminary section. The filling movement should be carried out with the highest possible velocity within the indicated range. The optimum velocities within the ranges described must be determined separately for every component geometry.
The pressure of recompacting results from the velocity of the casting plunger during the filling movement and from the casting-plunger displacement during the filling movement. When using the process according to the invention, the filling movement starts later than in the conventional pressure die-casting process, and accordingly the maximum pressure achieved during the recompacting is lower than in the conventional pressure die-casting process. It is generally between 600 bar and 1200 bar, in most cases between 700 bar and 900 bar; the highest possible pressure should be aimed for in order to achieve good infiltration.
In the process according to the invention, particularly when using aluminum or magnesium alloys, the temperature of the casting metal is between 680xc2x0 C. and 780xc2x0 C. The temperature should be selected to be as high as possible, so that during the filling of the impression and in particular during the infiltration of the insert the casting metal remains sufficiently hot for its temperature to be above the liquidus temperature, i.e. remains in liquid form and no solidification commences, which could cause the pores of the insert to become blocked. If the casting metal consists of an aluminum alloy, at temperatures of over 740xc2x0 C. the metal takes up hydrogen from the air, which has an adverse effect on the quality of the component which is to be cast therefrom. For this reason, the optimum temperature of the casting metal is between 700xc2x0 C. and 740xc2x0 C.
Also in order to prevent solidification of the casting metal prior to infiltration, it is advantageous to preheat the insert at a temperature of between 500xc2x0 C. and 800xc2x0 C. A preheating temperature which is between 600xc2x0 C. and 700xc2x0 C. is particularly advantageous, since this prevents the possibility of a chemical reaction between the casting metal and the insert and, at the same time, delays solidification of the casting metal.
The preheating of the insert may take place in an electrically heated chamber furnace, which is expedient when producing components in small numbers. However, when using series production, a continuous furnace is particularly suitable. This ensures a continuous supply of the inserts required for production and, moreover, allows a constant temperature of the inserts to be established. As the process sequence continues, the inserts can be picked up by a casting robot and placed into the die. This saves time over manual insertion and ensures that the insert is positioned accurately in the die.
For application of the process according to the invention, it is particularly advantageous for the casting metal used to be aluminum or magnesium or alloys of these metals. These metals have a low density and are particularly suitable for casting using the pressure die-casting process.
The insert is infiltrated particularly well by the casting metal if it has a porosity of between 30% and 80%, and very good infiltration can be achieved in particular at a porosity of 50%, the insert having a relatively high strength. The optimum pore diameter of the insert is between 1 xcexcm and 100 xcexcm, preferably is 20 xcexcm.